An edge light-type surface light source device which employs a light guide plate is widely used, mainly as a backlight device for a liquid crystal display.
It has become mainstream to employ the edge light-type surface light source device as a backlight device for a liquid crystal display, since the edge light-type surface light source device, in which a linear light source is provided at an edge of the light guide plate and linear light emitted from the linear light source is converted by the light guide plate into planar light, is more effective in reducing a thickness of a backlight device module for the liquid crystal display or of a product to which the backlight device module is applied, as compared with a direct backlight device which uses no light guide plate and a light source of which is disposed directly below a liquid crystal panel (see, for example, Patent Literature 1). Further, the edge light-type surface light source device is used for illumination in some cases.
Conventionally, it was mainly a cold cathode fluorescent lamp (CCFL) that was used as a light-emitting source of these light source devices, but in recent years, the cold cathode fluorescent lamp is increasingly replaced by a light-emitting diode (LED). The replacement enables to (i) abolish the use of environmentally-unfriendly mercury which is used in a CCFL or a fluorescent lamp, (ii) reduce power consumption, (iii) enhance color reproducibility, and (iv) lengthen the lives of the light source devices.
The following description will discuss, with reference to FIGS. 14 through 25, a conventional edge light-type surface light source device. FIG. 14 is an exploded perspective view illustrating an arrangement of a conventional edge light-type surface light source device. FIG. 15 is a cross-sectional view of the conventional edge light-type surface light source device illustrated in FIG. 14 and illustrates a state in which the conventional edge light-type surface light source device is assembled.
As illustrated in FIGS. 14 and 15, a LED light source device 100, which is a conventional edge light-type surface light source device, includes a housing 160, a light guide plate 120, a reflecting sheet 130, a diffusing sheet 150, and an LED light source substrate 140.
Note that in a case where the light guide plate 120 is relatively thin, the light guide plate 120 may be called a light guide sheet. Choice between the term light guide plate and the term light guide sheet is idiomatic, and there is no rigid distinction between the terms. The member called ‘light guide plate 120’ here denotes a light guide section in general, including a member called a light guide sheet.
The LED light source substrate 140 emits light to be applied to the light guide plate 120. The light applied from the LED light source substrate 140 enters an inside of the light guide plate 120 through an incident surface of the light guide plate 120, the incident surface being a side surface of the light guide plate 120. The light having entered the light guide plate 120 through the incident surface is subjected to mixing and homogenization inside the light guide plate 120 so as to be turned into planar light and emitted from a top surface of the light guide plate 120, the top surface being an irradiation surface of the light guide plate 120.
The reflecting sheet 130 is provided on a rear surface side (on an opposite side of the irradiation surface) of the light guide plate 120, and contributes to improvement of light use efficiency by causing light leaking to the rear surface side to travel back into the light guide plate.
The diffusing sheet 150 is provided on a front surface side (on a side of the irradiation surface) of the light guide plate 120, and has an effect of reducing luminance unevenness by homogenizing light emitted to the front surface side. The diffusing sheet 150 is used in combination with various other optical sheets (e.g., a lens sheet, a polarized light reflecting sheet, and the like), if necessary.
The housing 160 houses the above-described members such that the members are fixed and supported inside the housing 160.
By having the above-described arrangement, the LED light source device 100 serves as a surface irradiation device which uses light emission from the LED light source substrate 140.
Next, the following description will discuss, with reference to FIGS. 16 through 19, a specific arrangement of an LED light source substrate included in a conventional edge light-type surface light source device.
FIG. 16 shows an outer appearance of an LED light source substrate included in a conventional edge light-type surface light source device. FIG. 17 is a cross-sectional view of the LED light source substrate illustrated in FIG. 16.
As illustrated in FIG. 16, an LED light source substrate 600 is constituted by a flat wiring board 610, a plurality of LED packages 620 mounted on the flat wiring board 610, and a connector 601 also mounted on the flat wiring board 610. The plurality of LED packages 620 are electrically connected to the outside (not shown) via the connector 601 and a harness (not shown), and this arrangement allows light emission from the LED packages 620 to be externally controlled.
With reference to FIG. 17, a structure around each of the plurality of LED packages 620 will be described in further detail.
The wiring board 610 is constituted by a base 611, a wiring layer 612, and a solder resist layer 613 which are stacked on top of one another. An LED package 620 is connected to and fixed onto the wiring layer 612 by use of solder 626.
The LED package 620 includes an LED element 621, sealing resin 622, a bonding wire 623, a wiring layer 624, and a base 625. The LED element 621 is mounted on the base 625, and is connected to the wiring layer 624 by means of the bonding wire 623. The sealing resin 622 seals an inside of the base 625 to thereby protect parts inside the base 625 and connection between the parts. Further, the sealing resin 622 can contain phosphor, so that a color of light emitted from the LED element 621 can be changed. For example, an LED package emitting white light can be provided by using a blue LED element and yellow phosphor. The wiring layer 624 connects between (i) a portion of the LED package 620 which portion is connected to the solder 626 and (ii) a portion of the LED package 620 to which portion the LED element 621 is wire-bonded.
In the example illustrated in FIG. 17, the wiring layer 624 has a shape penetrating through the base 625, the solder 626 is connected to a portion of the wiring layer 624 which portion is on a bottom surface side of the base 625, and the LED element 621 is connected to a portion of the wiring layer 624 which portion is on a top surface side of the base 625.
The arrangement illustrated in FIG. 17 allows the LED element 621 to be electrically connected with the outside (not shown) via the wiring board 610, the connector 601, and the harness (not shown) while being fixed structurally. This allows light emission by the LED element 621 to be controlled externally.
FIG. 18 is another example of an LED light source substrate included in a conventional edge light-type surface light source device. FIG. 19 is a cross-sectional view of the LED light source substrate illustrated in FIG. 18, taken along a line indicated by an arrow A-A of FIG. 18. An LED light source substrate 500 illustrated in FIGS. 18 and 19 is arranged such that LED elements 515 are mounted on a base 511 by COB (Chip On Board) without using an LED package. That is, the LED elements 515 are directly mounted on the base 511. The base 511 can be provided with another layer (e.g., a wiring layer 513) on a surface of the base 511, and in this case, the LED elements 515 can be mounted on a surface of the another layer. In any case, according to COB, the LED elements 515 are directly mounted as they are onto the wiring board, instead of being stored in a package and indirectly mounted on the wiring board.
The base 511 has a front surface (a surface of the base 511 which surface horizontally extends and is located the closest to the top side of the sheet of in FIG. 19) and concave sections which are recessed from the front surface. The LED elements 515 are mounted within the concave sections.
In the LED light source substrate 500, the wiring layer 513 is electrically connected to the LED elements 515 via a bonding wire 516. Further, although not shown, the wiring layer 513 is electrically connected to an electrode terminal included in the connector 512. According to this arrangement, light emission from the LED elements 515 can be controlled by electrically controlling a harness (not shown) connected to the connector 512.
The LED elements 515, the bonding wire 516, and portions where the LED elements 515 are connected to the boding wire 516 are easily broken when an impact is applied. In order to prevent the breakage, the LED elements 515, the bonding wire 516, and the connection portions are sealed with sealing resin 514. That is, the concave sections are filled with the sealing resin 514. This arrangement allows the LED elements 515 and the bonding wire 516 to not only withstand a certain degree of externally applied impact but also be protected from water, a foreign matter, and the like.
Further, a color tone of light emitted from the LED light source substrate 515 can be adjusted by adding a colorant or phosphor to the sealing resin 514. For example, in a case where the LED elements 515 emit blue-colored light or ultraviolet rays and the sealing resin 514 contains suitable phosphor, the LED light source substrate 515 can emit white light.
Designing the LED light source substrate 140 to be constituted by LED packages and a wiring board as in the LED light source substrate 600 has such advantages that (i) a relatively large-sized substrate can be easily manufactured since an outer shape can be formed by pressing or routering and (ii) the LED packages can be mounted with use of a generally used mounter. On the other hand, mounting LED elements by COB as in the LED light source substrate 500 has such advantages that (i) the lack of a need to use solder in the mounting process eliminates temperature restrictions which may otherwise be imposed due to soldering temperatures at the time of using the solder and (ii) since the LED light source substrate can be manufactured into its final form by the same process as that for manufacturing an LED package, the LED light source substrate can be manufactured at low cost provided that the LED light source substrate has a small size.
FIG. 20 is a view illustrating a pattern of reflection of light in a conventional edge light-type surface light source device. In FIG. 20, light emitted from the LED light source substrate 140 enters the light guide plate 120 through the incident surface (a left side which is shown on the left on the sheet of FIG. 20) of the light guide plate 120. The light guide plate 120 is constituted by a light guide body 121 and reflection patterns 122.
In FIG. 20, a representative trace of incident light is indicated by arrows. Light emitted from the LED light source substrate 140 and applied to the incident surface of the light guide body 121 (i) enters an inside of the light guide body 121 while being refracted, in a case where an incident angle of the light is smaller than a certain degree and (ii) is totally reflected by the incident surface instead of entering the inside of the light guide body 121, in a case where the incident angle is larger than the certain degree.
The incident light having entered the light guide body 121 is repeatedly totally reflected by a top surface and a bottom surface of the light guide body 121. When the incident light hits a reflection pattern 122, the incident light is reflected so as to be diffused, so that many components are emitted from the top surface, i.e., an exit surface.
Normally, the reflection patterns 122 are set appropriately in order to achieve a homogenous emission pattern of surface light, a desired emission pattern of surface light, etc. For example, in order to achieve a homogenous emission pattern, the reflection patterns 122 are set so that a density of the reflection patterns is high at a position far away from the light source ((i) each reflection pattern is large, (ii) the number of reflection patterns per area is large, (iii) a combination of (i) and (ii), or the like), whereas the density of the reflection patterns is low at a position near the light source ((i) each reflection pattern is small, (ii) the number of reflection patterns per area is small, (iii) a combination of (i) and (ii), or the like).
The light guide body 121 is often made of a material such as an acrylic resin which has a very high transmittance, or polycarbonate which has a relatively high transmittance and a high strength. In particular, in a surface light source module having a relatively large size, the light guide body 121 is often made of acrylic resin, since an amount of light which is lost by being absorbed by the light guide plate is considerable in such a surface light source module. In contrast, in a case where the light guide body 121 has a relatively small size and requires strength, the light guide body 121 is often made of polycarbonate.
The reflection pattern 122 can be added to the light guide body 121 by, for example, laser marking the light guide body 121 or applying a coating material to the light guide body 121, or can be realized as a shape that is formed at the same time as molding the light guide body 121.
The following description will discuss, with reference to FIGS. 21 through 24, a positional arrangement of a light source substrate in a conventional edge light-type surface light source device. Each of FIGS. 21 through 24 schematically illustrates a positional arrangement of a light source substrate in a conventional edge light-type surface light source device.
In the example illustrated in FIG. 21, an LED light source substrate 140a and an LED light source substrate 140b are provided at respective ones of a pair of long sides (an upper side and a lower side which are shown at the top and the bottom, respectively, on the sheet of FIG. 21) of the light guide plate 120. Each of the LED light source substrate 140a and the LED light source substrate 140b has a length equal to that of a corresponding one of the pair of long sides of the light guide plate 120.
In the example illustrated in FIG. 22, an LED light source substrate 140a and an LED light source substrate 140b are provided at respective ones of a pair of short sides (a left side and a right side which are shown on the left and the right, respectively, on the sheet of FIG. 22) of the light guide plate 120. Each of the LED light source substrate 140a and the LED light source substrate 140b has a length equal to that of a corresponding one of the pair of short sides of the light guide plate 120.
In the example illustrated in FIG. 23, an LED light source substrate 140 is provided at one long side (a lower side which is shown at the bottom on the sheet of FIG. 23) of the light guide plate 120. The LED light source substrate 140 has a length equal to that of the one long side of the light guide plate 120.
In the example illustrated in FIG. 24, an LED light source substrate 140 is provided at one short side (a left side which is shown on the left on the sheet of FIG. 24) of the light guide plate 120. The LED light source substrate 140 has a length equal to that of the one short side of the light guide plate 120.
Note here that a total length of a light source substrate can be made shorter by providing the light source substrate at a short side of the light guide plate than providing the light source substrate at a long side of the light guide plate. Further, a total length of a light source substrate can be made shorter by providing the light source substrate at one side of the light guide plate than providing the light source substrate at two sides of the light guide plate.
For example, a total length of a light source substrate can be made shorter in the arrangement illustrated in FIG. 22 than in the arrangement illustrated in FIG. 21. Further, a total length of a light source substrate can be made shorter in the arrangement illustrated in FIG. 23 than in the arrangement illustrated in FIG. 21. Further, a total length of a light source substrate can be made shorter in the arrangement illustrated in FIG. 24 than in the arrangement illustrated in FIG. 22.
In general, reducing a total length of a light source substrate has many advantages such as a reduction in production cost, a reduction in weight of the product, a reduction of environmental burdens achieved by a reduction in volume of the members used, and a reduction in transportation cost which is achieved by a reduction in size and weight.
However, even in a case where the arrangement illustrated in FIG. 24 which enables the greatest reduction in total length of a light source substrate is employed, a length of the light source substrate needs to be equal to that of a corresponding side. This is because it is necessary to meet a demand for a light guide plate having as homogenous a luminance as possible, and the demand can be easily met by designing a light source substrate to have a length equal to that of a corresponding side. That is, in the case where the arrangement illustrated in FIG. 24 is employed, setting the length of the light source substrate shorter than that of the corresponding side may cause the light guide plate to have a portion having an insufficient luminance.
This issue will be described specifically with reference to FIG. 25. FIG. 25 is a view illustrating an irradiation area (irradiation region) which is irradiated with light from a light source substrate in a conventional surface light source device in which the light source substrate is provided at one side of a light guide plate. FIG. 25 illustrates an example in which the LED light source substrate 140 having a length shorter than that of one short side of the light guide plate 120 is experimentally provided at the one short side in a conventional surface light source device.
As illustrated in FIG. 25, in the conventional surface light source device, light emitted from the LED light source substrate 140 travels toward a right side of the light guide plate 120, and an irradiation area 210a which is irradiated with the light extends toward an upper side of the light guide plate 120 so as to form an angle α of refraction and extends toward a lower side of the light guide plate 120 so as to form an angle α of refraction.
This is because the light emitted from the LED light source substrate 140 is refracted by a side surface (i.e., a boundary surface) of the light guide plate 120. Accordingly, as shown in FIG. 25, a dark portion (a portion not indicated by hatching) which is not irradiated with the light from the LED light source substrate 140 is formed at each of an upper left corner section and a lower left corner section of the light guide plate 120.
In a case where the length of the LED light source substrate 140 is thus made shorter than that of the corresponding side, light can be directly applied to the irradiation area 210a but cannot be directly applied to the dark portion. This prevents the light guide plate in the conventional surface light source device to have a sufficient luminance. Therefore, the length of the LED light source substrate 140 cannot be designed shorter than that of the corresponding short side in the conventional surface light source device.
Even if the length of the LED light source substrate 140 is not designed shorter, an entire region of the light guide plate 120 in an original size can be an irradiation area by extending a length of a long side of the light guide plate 120. However, it is normally unacceptable that an extended portion of the long side exceeds 10% of a length of a short side of the light guide plate 120.
For example, in a case where the light guide plate 120 is made of an acrylic resin (refractive index: 1.49), the light will have a critical angle α of approximately 42°. Some types of optical glass have a refractive index lower than 1.49, for example, approximately 1.43, and in this case, the critical angle α is approximately 45°. In this case, if the length of the LED light source substrate 140 is less than 0.8 times the length of the corresponding short side, the length of the extended portion of the long side undesirably exceeds 10% of the length of the short side. Therefore, it is very difficult to set the length of the LED light source substrate 140 to be not more than 0.8 times the length of the corresponding short side.
However, there is still a demand for designing a length of a light source substrate to be shorter than that of a corresponding side. Conventionally, in order to meet the demand, art for setting a length of a light source substrate to be shorter than that of a corresponding side has been devised.
For example, Patent Literature 2 below discloses an arrangement in which, a length of a light source is shorter than that of a short side of a light guide plate, while an illumination light introduction section is provided so that illumination light emitted from the light source is widened by the illumination light introduction section so as to be guided to the light guide plate.
Further, Patent Literature 3 below discloses an arrangement in which, a length of a light source is shorter than that of a short side of a light guide plate, while a light scattering hole is formed in the light guide plate so that light is diffused in the light guide plate.
Furthermore, Patent Literatures 4 and 5 below each disclose an arrangement in which an L-shaped light source is provided at a corner section of a light guide plate, so that both a homogenous display luminance and a reduction in power consumption of the light source is achieved.
Further, Patent Literatures 6 and 7 each disclose an arrangement related to an illumination device employing an edge light-type surface light source device.